when you are on the toilet and you finally release a larger than normal lump of coagulated feses, and the resulting splash from the toilet bowl projects up and into your feses softened anal cavity, resulting in a jumping or startling motion and the flexing of muscle tissue in the arms and legs.

i was feeling really good because i was holding in this gigantic poop all day, then right as i shit i got a massive anal shock. i probably twitched for about a minute afterwards.